A thermal printer operates by sequentially heating desired patterns of small discrete areas ("pixels") of a thermal medium to produce desired light and dark patterns on the thermal medium. In some instances, the thermal medium can be a thermally sensitive medium which is heated directly, while in other instances, the thermal medium can be a thermal transfer ribbon which is heated to cause a small amount of dyed wax to be transferred to a medium which is not thermally sensitive.
The discrete areas of the thermal medium are heated by a thermal printhead which includes an array of minute, closely spaced resistive dots (or thermal print elements) that can be individually thermally controlled by means of electrical signals. The thermal medium is stepped past the printhead as each desired linear pattern is printed. The printhead is positioned over each part of the thermal medium for a predetermined interval of time (the "scan line time," SLT) which depends upon the printer's print speed.
In response to the print command signal, each thermal print element in a printhead receives an electrical energization signal that is a composite of two other electrical signals. Specifically, the energization signal is a logical AND of a strobe signal and a data signal. The strobe signal, which is periodically sent to each of the thermal print element is tailored to cause the thermal print element to reach and maintain a temperature within a prescribed temperature range under controllable conditions. The strobe signal typically consists of two portions--an initial "burn" time and a subsequent "chopped" time. If the strobe signal were applied directly to the thermal print element, the burn time portion of the strobe signal would force the thermal print element to heat up quickly. The chopped time portion of the strobe signal typically maintains the thermal print element's temperature and consists of approximately 25 cycles of a square wave with a 50 percent duty cycle. The data signal determines whether, within the period of the strobe signal, any portion of the strobe signal should be applied to a thermal print element to cause it to print.
In the past, it was known to adjust the strobe signal to account for the temperature of the printhead. For example, when a printer first begins operation, its printhead is still at ambient temperature and its individual thermal print elements must be given more energy to cause them to print. Therefore the burn time portion of the strobe signal can be lengthened so that the individual thermal print elements will be heated more and the printhead will reach a normal operating temperature. After the printhead has reached its operating temperature the strobe signal can be readjusted for these "normal" conditions.
To maintain high print quality under all of the possible operating conditions, it is necessary to keep the electrical potential across the printing elements in the printhead very well controlled. Presently, the electrical potential is measured between a pin in the connector from the supply printhead drive voltage (V.sub.sct =COM) and a pin in the connector from the printhead ground voltage (V.sub.gnd =GND). This potential is used by a sensing circuit in the power supply for the printhead to adjust the power supply voltage. In particular, the sensing circuit adjusts for variations in V.sub.sct through a feedback circuit in order to keep to keep V.sub.sct relatively constant at the printing elements.
A drawback of the present art is that voltage fluctuations internal to the printhead but not transmitted to the connector pins are not detected. Such internal fluctuations are especially prevalent during periods of heavy printing where many printing elements are turned on simultaneously. These fluctuations have been determined by the inventors to be significant because they degrade the quality of the printing produced by the printhead.